Wrath of Poseidon by Clive Cussler

Treasure hunters Remi and Sam Fargo are in Washington, DC looking for the final clue in a puzzle more than a decade old - a dangerous mystery which originally led to their teaming up.

For ten years ago, a chance meeting that threw Remi and Sam together led to the pair diving together for ancient sunken wrecks off the Greek coast.

But when a secretive billionaire decided he didn't want anyone diving in the seas near his operations, the pair found themselves in deep trouble. Only now, returning ten years later, do they finally understand who and what they were up against.

For with the lost riches of Croesus lying somewhere in these waters, their formidable enemy won't be giving up easily.

Suddenly, finding the treasure becomes less important than making it home alive . . .